Implantable medical electrical stimulation and/or sensing leads (electrical medical leads) are well known in the fields of tissue stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation, and in other fields of electrical stimulation or monitoring of electrical signals or other physiologic parameters. In the field of cardiac stimulation and monitoring, the electrodes of epicardial or endocardial cardiac leads are affixed against the epicardium or endocardium, respectively, or inserted therethrough into the underlying myocardium of the heart wall.
The lead body of a cardiac lead typically comprises one or more insulated conductive wire surrounded by an insulating outer sheath. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. The proximal lead connector elements of permanently implantable epicardial and endocardial cardiac leads are designed to be coupled to a pacemaker or defibrillator implantable pulse generator (IPG) or an implanted monitor and to be chronically implanted in the patient's body.
Cardiac leads having a single stimulation and/or sensing electrode at the lead distal end, a single conductor, and a single connector element are referred to as unipolar cardiac leads. Cardiac leads having two or more stimulation and/or sensing electrodes at the lead distal end, two or more respective conductors, and two or more respective connector elements are referred to as bipolar lead or multi-polar leads, respectively.
Epicardial or myocardial cardiac leads, or simply epicardial leads, are implanted by exposure of the epicardium of the heart typically through a limited thorocotomy or a more extensive surgical exposure made to perform other corrective procedures. Endocardial cardiac leads, or simply endocardial leads, are implanted through a transvenous route to locate one or more sensing and/or stimulation electrode along or at the distal end of the lead in a desired implantation site in a chamber of the heart or a blood vessel of the heart. It is necessary to accurately position the electrode surface against the endocardium or within the myocardium or coronary vessel at the implantation site. An active or passive fixation mechanism is typically incorporated into the distal end of permanent cardiac leads and is deployed at the implantation site to maintain the distal end electrode in contact with the endocardium or within the myocardium. Commonly employed active fixation mechanisms comprise a distal fixation helix that is rotated to be screwed into the myocardium and that may function as apace/sense electrode. Commonly employed passive fixation mechanisms comprise a plurality of soft pliant tines that lodge in trabeculae or against a coronary vessel wall or a particularly shaped distal segment of the lead body, e.g., that disclosed in commonly assigned U.S. Pat. No. 5,999,858, that causes the distal pace/sense electrode(s) to bear against a heart chamber wall or coronary vessel wall.
The heart beats approximately 100,000 times per day or over 30 million times a year, and each beat stresses at least the distal end segment of an implanted permanent endocardial lead. The lead conductors and insulation are subjected to cumulative mechanical stresses, as well as material reactions, over the years of implantation that can result in degradation of the insulation or fractures of the lead conductors with untoward effects on device performance and patient well being. The endocardial lead body is subjected to continuous flexing as the heart contracts and relaxes and is formed to be highly supple, flexible and durable. Over the last 30 years, it has become possible to reduce endocardial lead body diameters from 10 to 12 French (3.3 to 4.0 mm) down to 2 French (0.66 mm) presently through a variety of improvements in conductor and insulator materials and manufacturing techniques.
Such a small diameter endocardial lead body lacks the column stiffness necessary to push it through the twists and turns of the venous pathway or vasculature into the right atrium and then to the desired implantation sites in a right heart chamber or within the coronary sinus or a branching cardiac vein. Historically, it has been necessary to temporarily stiffen the lead body to advance the lead distal end through the transvenous pathway and to locate the distal electrode(s) at the desired implantation site either by use of stiffening stylet inserted into a lead body lumen or by advancing the lead body through the lumen of a guide catheter or over a guidewire advanced to the site, or both. Stiffening stylets are typically formed as a single elongated wire in which a distal curve can be manually formed by the implanting physician to induce a like bend in the lead body to facilitate navigation through the vasculature and to aim the distal electrode and/or fixation mechanism against the endocardium or into a coronary vessel. Other stiffening stylets include a handle and pull wire enabling selective changes in curvature of a stylet distal segment while the stylet wire is within the lead body lumen to facilitate such navigation. Certain stiffening stylets comprise an stylet sheath and stylet wire that is movable axially within the stylet sheath, so that the curved distal segment of the stylet wire can be selectively advanced from or retracted into the stylet sheath to facilitate such navigation as disclosed in U.S. Pat. No. 5,728,148, for example.
Various guide catheters typically comprising a guide catheter hub and an elongated guide catheter body have also been proposed for introducing an endocardial lead into the coronary sinus to dispose one or more electrode at an implantation site in the coronary sinus or a branching vessel. Simple single lumen or relatively more complex guide catheters used in certain instances with a stiffening stylet or guidewire are disclosed in commonly assigned U.S. Pat. Nos. 5,246,014, 5,897,584, 6,280,433, 6,379,346 and, 6,408,214, for example. In at least certain embodiments, the guide catheter body is formed to be manually separable or slittable along its length by a slitting tool to aid in removing the guide catheter from the endocardial lead body introduced through a guide catheter lumen after the electrode(s) is disposed at the desired implantation site, and any fixation mechanism is affixed.
A still further technique of implantation of such miniaturized, highly flexible, endocardial leads employs a guidewire that is first advanced through the tortuous transvenous pathway. The endocardial lead is then advanced through the pathway alongside or over the guidewire as disclosed in U.S. Pat. Nos. 5,003,990, 5,304,218, 5,902,331, 6,132,456, and 6,185,464, for example. Some of these techniques require that the lead body be configured to provide an over-the-wire connection and possess sufficient column strength to be advanced over the guidewire. For example, a lead body lumen extends from a proximal lumen opening to a distal lumen opening, and the guidewire is inserted through the lead body lumen to provide over the wire advancement of the endocardial lead through the vasculature, the right atrium and the coronary sinus. Other techniques employ elongated pusher tools that have sufficient column strength applied against the lead body distal end and extending alongside the lead body and the over the guidewire. These techniques are relatively complex to execute. Moreover, the rotation of the active fixation helix at the lead distal end through rotation of the assembly can be problematic.
Further complications arise when a guide catheter is employed in the introduction procedure to facilitate advancement of such small diameter endocardial leads. When an over-the-wire technique is employed, the guidewire is first advanced through the skin incision, the vasculature, the right atrium, and into the coronary sinus. A guide catheter is introduced over the guidewire, and the lead is introduced over the guidewire and through the guide catheter lumen. When a stylet is employed, the guidewire is removed after the guide catheter is positioned, and the lead, stiffened by the stylet, is introduced through the guide catheter lumen. In either case, the distal electrode(s) is advanced out of the guide catheter distal lumen opening and advanced further into a vessel branching from the coronary sinus to an implantation site, and any distal fixation mechanism is affixed into the myocardium.
The subsequent removal of the stylet or guidewire and the guide catheter can impose forces on the lead body that detach the fixation mechanism and retract the electrode(s) from the desired implantation site. It is not possible to manually grip the proximal portion of the lead body extending out of the skin incision to hold it in place and counter these forces during withdrawal of the guide catheter and the stylet or guidewire as long as the lead body is within the guide catheter lumen. The guide catheter can be slowly retracted and slit or split along its length during the retraction to access the lead body, but detachment and electrode dislodgement can still occur.
It has been proposed in U.S. Pat. Nos. 6,356,791 and 6,671,560 to replace the guidewire or stylet with a specially shaped, removal wire that is longer than the guide catheter. The proximal connector assembly of the endocardial lead extends proximally from the guide catheter hub, and the lead removal wire distal end is inserted through the lead connector pin into the lead body lumen, whereby the removal wire distal end engages with the lead body lumen near the proximal connector pin. In a further approach embodied in the FINISHING WIRE® removal wire available from Guidant, Corp., Saint Paul, Minn., the removal wire is modified to have a tubular cap fixed in position proximal to the removal wire distal end that engages against the lead connector pin when the removal wire distal end is fully inserted into and engaged against the lead lumen. Force can be applied manually at the removal wire proximal end to hold the lead body proximal end stable as the guide catheter is retracted over the removal wire. The removal wire can be detached from the lead connector pin when the guide catheter is fully retracted onto the removal wire, revealing the lead connector pin. Thus, the removal wire must be somewhat longer than the guide catheter body and the length of the lead body lumen it is to be inserted into. Endocardial leads and guide catheters are marketed having a variety of lengths, resulting in the necessity of providing a removal wire tailored in length to the length of the lead body and guide catheter body. It is also necessary to fully seat the removal wire into the lead body lumen before the tubular cap engages the lead connector pin. It is desirable to maintain control of the connector pin even though the best position for the distal tip of the removal wire may not be at the distal tip of the lead. In certain cases, it may be desirable to only extend the removal wire part way through the lead body lumen resulting in an increased length of the removal wire extending proximally from the sleeve lumen. Such a length or removal wire can be unwieldy in the surgical field presenting difficulties in handling the lead, the removal wire, and the guide catheter during insertion of the removal wire into the lead body lumen and retraction of the guide catheter.
A further removal wire has been proposed in U.S. Pat. No. 6,625,496 that constitutes an extension sleeve that affixes to the outer surface lead connector pin. The extension sleeve has an outer diameter selected to fit within the guide catheter lumen and a lubricated outer surface, e.g., a hydrogel coating. In use, the guidewire or stylet employed to introduce the lead electrode(s) and fixation mechanism to the implantation site is removed, and the distal end of the extension sleeve is fitted to the lead connector pin. A long stylet or guidewire is extended through the axially aligned sleeve and lead body lumens so that it is seated or passes out of the distal lumen end opening and extends proximally of the extension sleeve apparently, to stabilize the lead body during removal of the guide catheter. The guide catheter is then retracted over the extension tube until the lead connector pin is exposed. Thus, the extension tube must be somewhat longer than the guide catheter body. In certain cases, it may be desirable to only extend the long stylet or guidewire part way through the lead body lumen resulting in an increased length of the stylet or guidewire extending proximally from the sleeve lumen. This length can also be unwieldy in the surgical field and present difficulties in handling the lead, the removal wire, and the guide catheter during insertion of the removal wire into the lead body lumen and retraction of the guide catheter.
Thus, a need remains for a system and method for introducing a small diameter cardiac lead lacking pushability and torqueability that enables advancement of the distal electrode through tortuous pathways into a wide variety of implantation sites in a heart chamber or in a coronary vessel of the left heart chambers and reliable fixation at the selected implantation site.